Add sycl_khr_group_interface extension

This extension introduces an alternative interface for groups of
work-items, offering several improvements over the SYCL 2020 interface:

- Shorter names for member functions, dropping the get_ prefix.

- Cleaner separation between properties of a group (e.g., a group id)
  and properties of the calling work-item (e.g., its id within a group).

- Clearer distinction between "group" concept and "work_group" class.

- New work_item class to represent a single work-item within a specific
  parent group. This class also satisfies the group concept, modeling
  a group containing a single work-item.

adoc/extensions/index.adoc

@@ -11,3 +11,5 @@ specification, but their design is subject to change.
 
 // leveloffset=2 allows extensions to be written as standalone documents
 // include::sycl_khr_extension_name.adoc[leveloffset=2]
+
+include::sycl_khr_group_interface.adoc[leveloffset=2]